Lithium manganese iron phosphate slurry, pole piece and preparation method of lithium manganese iron phosphate slurry

Abstract

The invention belongs to the technical field of lithium ion batteries, and particularly relates to lithium manganese iron phosphate slurry, a pole piece and a preparation method thereof.According to the preparation method, a lithium manganese iron phosphate material is added into a conductive adhesive solution in two steps, modified PVP is introduced as a dispersing agent, and under the dual action of steric hindrance and electrostatic repulsion, the lithium manganese iron phosphate slurry is prepared; the viscosity of the slurry can be effectively reduced, and meanwhile, the slurry is converted to a pseudoplastic fluid characteristic, so that the dispersity and the stability of the lithium manganese iron phosphate are remarkably improved. Besides, PVDF with large and small molecular weights is compounded, so that the stacking among the molecular weights is tighter, the intermolecular acting force between the PVDF and the powder is larger, the cohesiveness is better, and the problems of cracking, powder falling and the like in the coating process are reduced; meanwhile, complex groups (polar groups such as carboxyl, hydroxyl, amido and imide) on a large-molecular-weight PVDF long chain can form chemical bonds with the foil, so that the cohesiveness of the foil is further improved, and the flexibility of the pole piece is also improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion batteries, and in particular relates to a lithium iron manganese phosphate slurry, a pole piece and a preparation method thereof. Background Art

[0002] Lithium iron manganese phosphate cathode materials have attracted widespread attention due to their high voltage platform and moderate theoretical specific capacity. However, due to the presence of manganese, their electronic and ionic conductivity are poor. Therefore, commercially, nano-particles, carbon coatings, and doping are often used to improve the conductivity of lithium iron manganese phosphate materials. However, the resulting reduction in particle size and increase in specific surface area have a greater impact on the processing performance of lithium iron manganese phosphate materials. The existing homogenization process, namely the traditional wet and dry homogenization process, is difficult to disperse it completely, and it is easy to have the problem of particle dispersion difficulty, resulting in difficulty in sieving the slurry, and particles and scratches on the coating; at the same time, there will be too much glue absorption, resulting in reduced adhesion between particles and between particles and foil, poor flexibility of the pole piece, and large membrane resistance, which seriously affects the performance of lithium iron manganese phosphate batteries.

[0003] In view of this, the present invention is proposed. Summary of the invention

[0004] In order to solve the deficiencies and shortcomings of the prior art, the present invention provides a lithium iron manganese phosphate slurry, a pole piece and a preparation method thereof. During preparation, the main material is added in steps, and two polyvinylidene fluoride (PVDF) with different molecular weights are compounded as a binder, supplemented with an appropriate amount of dispersant, to obtain a lithium iron manganese phosphate slurry with high solid content, small fineness, good dispersibility and stability, and then by adjusting the discharge viscosity, a lithium iron manganese phosphate pole piece with stable surface density, good flexibility, adhesion and conductivity is obtained. The problem of difficulty in dispersing the slurry particles of lithium iron manganese phosphate positive electrode materials, low adhesion, poor pole piece flexibility and large membrane resistance is solved.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] In a first aspect, a method for preparing lithium manganese iron phosphate slurry comprises the following preparation steps:

[0007] (1) mixing a binder and a part of a dispersant in a solvent to prepare a glue solution, wherein the binder includes PVDF-1 and PVDF-2, the PVDF-1 is a polyvinylidene fluoride with a molecular weight of 50 to 90W, the PVDF-2 is a modified polyvinylidene fluoride with a molecular weight of 120 to 150W containing a carbonyl polar functional group, and the dispersant is a modified PVP;

[0008] (2) adding graphene, carbon nanotubes and conductive carbon black as conductive agents to the glue obtained in step (1) in sequence and mixing them to obtain a conductive glue;

[0009] (3) adding part of the lithium iron manganese phosphate to the conductive glue obtained in step (2) and mixing them evenly; then adding the remaining lithium iron manganese phosphate, dispersant and solvent to the slurry and mixing them evenly to obtain lithium iron manganese phosphate slurry.

[0010] Furthermore, the carbonyl polar functional group includes one or more of a carboxyl group, a hydroxyl group, an amide group, and an imide group;

[0011] And / or, the mass ratio of PVDF-1 to PVDF-2 is 1:3-9;

[0012] And / or, the modified PVP is a polyvinyl pyrrolidone solution modified by a fluorinated acrylic ester copolymer or an alcohol amine substance;

[0013] And / or, the mass ratio of the conductive carbon black, carbon nanotubes and graphene is (1-1.5): (0.4-0.5): (0.1-0.5);

[0014] And / or, the lithium manganese iron phosphate: the carbon coating amount accounts for 1.6-1.8wt% of the lithium manganese iron phosphate, the particle size D50 is 400-500nm, the particle size D90 is 0.8-1μm, and the specific surface area is 15-20m 2 / g;

[0015] And / or, the lithium iron manganese phosphate slurry includes powder and solvent, and the mass ratio of the powder to the solvent is (55:45) to (63:37).

[0016] Furthermore, the powder is composed of 94.6-97% lithium manganese iron phosphate, 1-3% conductive agent, 0.8-2% binder, and 0.05-0.4% dispersant by mass percentage;

[0017] and / or, the viscosity of the modified PVP is less than 200 mPa·s, and the pH is between 10 and 12;

[0018] And / or, the solvent is N-methylpyrrolidone.

[0019] Furthermore, in step (1), the solid content of the glue solution is 4-6%;

[0020] And / or, in step (3), the viscosity of the lithium iron manganese phosphate slurry is 3000-8000 mPa·s.

[0021] Furthermore, in steps (1) and (3), the mass ratio of the dispersant added twice is (1:3) to (1:1);

[0022] And / or, in step (3), the mass ratio of the lithium manganese iron phosphate added twice is (3:1) to (1:1).

[0023] Furthermore, in step (1) and / or step (2) and / or step (3), the mixing includes first slowly stirring the scraping material and then dispersing it at a high speed.

[0024] Furthermore, the revolution speed and rotation speed of the slow stirring scraper are 10-15 rpm and 1000-1500 rpm respectively, and the time is 5-10 min;

[0025] And / or, the revolution speed and rotation speed of the high-speed dispersion are 40-55 rpm and 3500-5500 rpm respectively, and the time is 1-3 hours.

[0026] In a second aspect, a lithium iron manganese phosphate slurry is prepared by the preparation method described in the first aspect.

[0027] In a third aspect, a lithium iron manganese phosphate pole piece is provided, wherein the lithium iron manganese phosphate slurry described in the second aspect is coated on a carbon-coated aluminum foil of a certain thickness at a certain surface density, dried, and then rolled at a certain compaction density to obtain the lithium iron manganese phosphate pole piece.

[0028] Furthermore, the surface density is 350-380 g / m 2 ;

[0029] and / or, the thickness is 12 to 14 μm;

[0030] And / or, the compacted density is 2.2 to 2.4 g / cm 3

[0031] Compared with the prior art, the present invention has at least the following beneficial effects:

[0032] The preparation method of the present invention adds the lithium iron manganese phosphate material to the conductive glue in two steps, and then introduces the modified PVP as a dispersant. Under the dual effects of steric hindrance and electrostatic repulsion, the viscosity of the slurry can be effectively reduced, and the slurry is transformed into a pseudoplastic fluid characteristic, so that the dispersibility and stability of the lithium iron manganese phosphate are significantly improved. In addition, by using a large and small molecular weight PVDF compound, the molecular weights are stacked more tightly, so that the intermolecular force between it and the powder is greater, the adhesion is better, and the problems of cracking and powdering during the coating process are reduced; at the same time, the complexing groups (polar groups such as carboxyl, hydroxyl, amide, and imide) on the long chain of the large molecular weight PVDF can form chemical bonds with the foil, further improving its adhesion and also improving the flexibility of the pole piece. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme in the embodiment of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention. The process parameters of the following embodiments that do not specify specific conditions are usually based on conventional conditions.

[0034] The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed in the present invention.

[0035] According to a first aspect of the present invention, a method for preparing lithium manganese iron phosphate slurry comprises the following preparation steps:

[0036] (1) mixing a binder and a part of a dispersant in a solvent to prepare a glue solution, wherein the binder includes PVDF-1 and PVDF-2, the PVDF-1 is a polyvinylidene fluoride with a molecular weight of 50 to 90W, the PVDF-2 is a modified polyvinylidene fluoride with a molecular weight of 120 to 150W containing a carbonyl polar functional group, and the dispersant is a modified PVP;

[0037] (2) adding graphene, carbon nanotubes and conductive carbon black as conductive agents to the glue obtained in step (1) in sequence and mixing them to obtain a conductive glue;

[0038] (3) adding part of the lithium iron manganese phosphate to the conductive glue obtained in step (2) and mixing them evenly; then adding the remaining lithium iron manganese phosphate, dispersant and solvent to the slurry and mixing them evenly to obtain lithium iron manganese phosphate slurry.

[0039] The preparation method of the present invention adopts a complete wet process, adjusts the timing of adding the dispersant and lithium manganese iron phosphate, adds them multiple times in different amounts, first prepares the conductive glue and then adds the lithium manganese iron phosphate twice, which can fully exert the effect of the PVDF binder and avoid the problem of easy agglomeration of LMFP particles. Under the dual effects of steric hindrance and electrostatic repulsion, the modified PVP can effectively reduce the viscosity of the slurry, and at the same time, the slurry is transformed into a pseudoplastic fluid characteristic, so that the dispersibility and static stability of the lithium manganese iron phosphate are significantly improved.

[0040] Specifically, when preparing the glue solution first, part of the dispersant is used to disperse the mixed binder so that the binder is evenly dispersed in the solvent and can better play the bonding role. More specifically, the binder PVDF is a compound of polyvinylidene fluoride PVDF-1 with a molecular weight of 50 to 90W and modified polyvinylidene fluoride PVDF-2 with a molecular weight of 120 to 150W containing carbonyl polar functional groups. It not only has the bonding effect of PVDF-1 and PVDF-2 respectively, but especially the presence of carbonyl polar functional groups in PVDF-2 can effectively enhance the interaction between the positive electrode active material and the current collector, form chemical bonds with the foil, further improve its bonding, improve the flexibility of the pole piece, and its unique structure can also effectively prevent slurry gel; at the same time, it can also improve the pole piece. Adhesion and corrosion resistance in electrolyte; and the large and small molecular weight PVDFs are compounded into PVDF-1 / PVDF-2, and the molecular weights are stacked more tightly, which makes the intermolecular force between them and the powder stronger, and the adhesion is better, reducing the problems of cracking and powder falling during the coating process; and it can avoid the sedimentation caused by insufficient adhesion when using small molecular weight PVDF-1 alone and the gel phenomenon caused by agglomeration when using large molecular weight PVDF-2 alone, so as to ensure that the slurry has a high solid content and a high coating weight, taking into account the adhesion and flexibility of the electrode, and improving the coating cracking. When preparing the conductive glue, different conductive agents are added from difficult to easy according to the difficulty of dispersion of the conductive agent. The three conductive agents of graphene, carbon nanotubes and conductive carbon black are compounded to form an effective "point to line to surface" conductive network, so as to obtain a conductive glue in which the binder and the conductive agent are evenly dispersed.

[0041] When lithium manganese iron phosphate is dispersed later, lithium manganese iron phosphate material (LMFP particles) is added to the conductive glue in two steps to avoid the problem of easy agglomeration of LMFP particles. Specifically, some LMFP particles are added first, and the PVDF particles uniformly dispersed in the conductive glue will be adsorbed on the surface of the LMFP particles. Through the effects of PVDF-1 adhesion and / or PVDF-2 adhesion and / or PVDF-1 / PVDF-2 adhesion, the contact area and contact points between particles are increased, and the bonding performance is improved; at the same time, the multi-charge repulsion site effect of PVDF-1 and / or PVDF-2 and / or PVDF-1 / PVDF-2 and / or the unique structural effect of the polar functional groups of PVDF-2 can effectively prevent the slurry from gelling. Then the remaining LMFP particles and modified PVP are introduced, and the polar groups on the surface of the modified PVP are closely adsorbed on the surface of the LMFP particles, and together with the solvated chains at the other end, the properties of the surface of the LMFP particles are changed, thereby further reducing the agglomeration and sedimentation caused by the interaction between LMFP particles, and finally forming a slurry with good dispersibility and static stability. In addition, the conductive network established by the compound of three conductive agents is evenly distributed around the LMFP particles, which significantly improves the poor conductivity of LMFP caused by the presence of manganese elements, increases the conductivity of the electrode, and then improves the polarization of the battery, reduces the internal resistance of the battery, and significantly improves the rate performance of the battery.

[0042] As an optional embodiment of the preparation method of the present invention, the carbonyl polar functional group includes one or more of a carboxyl group, a hydroxyl group, an amide group, and an imide group;

[0043] And / or, the mass ratio of PVDF-1 to PVDF-2 is 1:3 to 9 (e.g., 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9);

[0044] And / or, the modified PVP is a polyvinyl pyrrolidone solution modified by a fluorinated acrylic ester copolymer or an alcohol amine substance;

[0045] And / or, the mass ratio of the conductive carbon black, carbon nanotubes and graphene is (1-1.5):(0.4-0.5):(0.1-0.5), specifically 1:0.4:0.5, 1.1:0.42:0.45, 1.2:0.44:0.35, 1.3:0.46:0.25, 1.4:0.48:0.15, 1.5:0.5:0.1.

[0046] And / or, the lithium manganese iron phosphate: the carbon coating amount accounts for 1.6-1.8wt% (such as 1.63wt%, 1.65wt%, 1.69wt%, 1.73wt%, 1.75wt%, 1.78wt%) of the lithium manganese iron phosphate, the particle size D50 is 400-500nm (such as 420nm, 440nm, 460nm, 480nm), the particle size D90 is 0.8-1μm (such as 0.83μm, 0.85μm, 0.87μm, 0.89μm, 0.93μm, 0.95μm, 0.98μm), and the specific surface area is 15-20m 2 / g(such as 16m 2 / g, 17m 2 / g, 18m 2 / g, 19m 2 / g);

[0047] And / or, the lithium iron manganese phosphate slurry includes powder and solvent, and the mass ratio of the powder to the solvent is (55:45) to (63:37), specifically 56:44, 57:43, 58:42, 59:41, 60:40, 61:39, and 62:38.

[0048] As an optional embodiment of the preparation method of the present invention, the powder is composed of 94.6-97% lithium manganese iron phosphate, 1-3% conductive agent, 0.8-2% binder, and 0.05-0.4% dispersant by mass percentage;

[0049] and / or, the viscosity of the modified PVP is less than 200 mPa·s, and the pH is between 10 and 12;

[0050] And / or, the solvent is N-methylpyrrolidone.

[0051] As an optional embodiment of the preparation method of the present invention, in step (1), the solid content of the glue solution is 4-6% (such as 4.3%, 4.6%, 4.9%, 5.2%, 5.5%, 5.7%);

[0052] And / or, in step (3), the viscosity of the lithium iron manganese phosphate slurry is 3000-8000 mPa·s (such as 3500 mPa·s, 4000 mPa·s, 4500 mPa·s, 5000 mPa·s, 5500 mPa·s, 6000 mPa·s, 6500 mPa·s, 7000 mPa·s, 7500 mPa·s).

[0053] As an optional embodiment of the preparation method of the present invention, in steps (1) and (3), the mass ratio of the two additions of the dispersant is (1:3) to (1:1), specifically 1:2.7, 1:2.5, 1:2.2, 1:2, 1:1.7, 1:1.5, 1:1.2;

[0054] And / or, in step (3), the mass ratio of the lithium manganese iron phosphate added twice is (3:1) to (1:1), specifically 2.7:1, 2.5:1, 2.2:1, 2:1, 1.7:1, 1.5:1, 1.2:1.

[0055] As an optional embodiment of the preparation method of the present invention, in step (1) and / or step (2) and / or step (3), the mixing includes first slowly stirring the material and then dispersing it at a high speed.

[0056] As an optional embodiment of the preparation method of the present invention, the revolution speed and rotation speed of the slowly stirred scraper are 10-15 rpm and 1000-1500 rpm respectively, and the time is 5-10 min;

[0057] And / or, the revolution speed and rotation speed of the high-speed dispersion are 40-55 rpm and 3500-5500 rpm respectively, and the time is 1-3 hours.

[0058] According to a second aspect of the present invention, a lithium iron manganese phosphate slurry is prepared by the preparation method described in the first aspect.

[0059] According to a third aspect of the present invention, a lithium iron manganese phosphate pole piece is provided. The lithium iron manganese phosphate slurry described in the second aspect is coated on a carbon-coated aluminum foil of a certain thickness at a certain surface density, dried, and then rolled at a certain compaction density to obtain the lithium iron manganese phosphate pole piece.

[0060] Furthermore, the surface density is 350-380 g / m 2 ;

[0061] and / or, the thickness is 12 to 14 μm;

[0062] And / or, the compacted density is 2.2 to 2.4 g / cm 3 .

[0063] The preparation of the slurry and the pole piece of the present invention can be achieved by using existing technical equipment. The equipment is not listed one by one here. The embodiment provides a lithium iron manganese phosphate slurry, a pole piece and a preparation method thereof. The preparation method of the lithium iron manganese phosphate slurry and the pole piece is as follows:

[0064] S1. First, add 50% of the binder, dispersant and solvent into a double planetary pulping machine in proportion and mix evenly, stir slowly for 5-10 minutes at an orbital speed of 10-15rpm and a rotational speed of 1000-1500rpm respectively, scrape the material, and then disperse at high speed for 1-3 hours at an orbital speed of 40rpm and a rotational speed of 3500rpm respectively, and a vacuum degree of -90KPa to obtain a glue solution with a solid content of 4-6%;

[0065] S2. Add graphene to the glue in proportion, stir slowly for 10 minutes at a revolution speed of 10-15 rpm and a rotation speed of 1000-1500 rpm, scrape the material, and then disperse at high speed for 1 hour at a revolution speed of 45 rpm and a rotation speed of 4500 rpm, a vacuum degree of -90 KPa, and then add carbon nanotubes to the glue in proportion, stir slowly for 10 minutes at a revolution speed of 10-15 rpm and a rotation speed of 1000-1500 rpm. , scrape the material, and then disperse at high speed for 1 hour at a revolution speed of 45rpm and a rotation speed of 4500rpm, a vacuum degree of -90KPa, and finally add the conductive carbon black to the glue according to the proportion, stir slowly for 10 minutes at a revolution speed of 10-15rpm and a rotation speed of 1000-1500rpm, scrape the material, and then disperse at high speed for 1.5 hours at a revolution speed of 45rpm and a rotation speed of 4500rpm, a vacuum degree of -90KPa, to obtain a conductive glue;

[0066] S3. Then, 75% of the lithium manganese iron phosphate is added to the conductive glue, and the mixture is slowly stirred for 5 to 10 minutes at an orbital speed of 10 to 15 rpm and a rotational speed of 1000 to 1500 rpm, and the mixture is scraped. Then, the mixture is dispersed at high speed for 1 to 2 hours at an orbital speed of 50 rpm and a rotational speed of 5500 rpm, and the vacuum degree is -90 KPa, and the mixture is mixed evenly. Finally, the remaining 25% of the lithium manganese iron phosphate and 50% of the dispersant and the solvent are added to the slurry in sequence, and the mixture is slowly stirred for 5 to 10 minutes at an orbital speed of 10 to 15 rpm and a rotational speed of 1000 to 1500 rpm, and the mixture is scraped. Then, the mixture is dispersed at high speed for 2 to 3 hours at an orbital speed of 50 rpm and a rotational speed of 5500 rpm, and the vacuum degree is -90 KPa, and the mixture is mixed evenly to obtain a lithium manganese iron phosphate slurry with a viscosity of 3000 to 8000 mPa·s.

[0067] S4, the slurry is adjusted to 350-380g / m 2 The surface density is coated on a 12-14 μm thick carbon-coated aluminum foil, dried, and then coated at 2.2-2.4 g / cm 3 The compaction density is rolled to obtain a lithium manganese iron phosphate electrode.

[0068] The lithium manganese iron phosphate slurry includes a powder and a solvent in a mass ratio of (55:45) to (63:37); based on the total mass of the powder, the powder is composed of 94.6% to 97% of lithium manganese iron phosphate, 1% to 3% of a conductive agent, 0.8% to 2% of a binder, and 0.05% to 0.4% of a dispersant; the conductive agent is a composite of conductive carbon black, carbon nanotubes, and graphene, and the mass ratio is (1 to 1.5): (0.4 to 0.5): (0.1 to 0.5); the binder is a composite of PVDF-1 and PVDF-2, the PVDF-1 is a polyvinylidene fluoride with a molecular weight of 50 to 90W, and the PVDF-2 is a modified polyvinylidene fluoride containing carbonyl polar functional groups with a molecular weight of 120~150W, the carbonyl polar functional group includes one or more of carboxyl, hydroxyl, amide, and imide, the mass ratio of PVDF-1 to PVDF-2 is 1:3~9; the dispersant is modified PVP, and the modified PVP is a polyvinyl pyrrolidone (PVP) solution modified by a fluorinated acrylate copolymer and an alcohol amine substance, with a viscosity of <200mPa·s, an effective ingredient of 20%, a pH of 10-12, and a moisture content of <1000ppm; the lithium manganese iron phosphate: the carbon coating amount accounts for 1.6~1.8wt% of the lithium manganese iron phosphate, the particle size D50 is 400~500nm, the particle size D90 is 0.8~1μm, and the specific surface area is 15~20m 2 / g; the solvent is N-methylpyrrolidone (NMP).

[0069] The embodiments of the present invention will be described in detail below in conjunction with specific examples and comparative examples. The following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. The PVDF-1 used in the examples and comparative examples is synthesized by the emulsion method, and the PVDF-2 is synthesized by the suspension method. The dispersant is the CGI-D37P product of Shenzhen Yanyi New Materials Co., Ltd., which has a viscosity of <200mPa·s, an active ingredient of 20%, a pH of 10-12, and a moisture content of <1000ppm. If other specific conditions are not specified, they will be carried out under normal conditions. The reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased through normal channels.

[0070] Example 1

[0071] This embodiment provides a lithium manganese iron phosphate slurry and a pole piece, and the preparation method thereof comprises the following steps:

[0072] S1. First, add PVDF-1, PVDF-2, 50% of the dispersant and NMP into a double planetary pulper in proportion and mix evenly. Stir slowly for 10 minutes at an orbital speed of 10 rpm and an autorotation speed of 1000 rpm respectively, scrape the material, and then disperse at high speed for 2 hours at an orbital speed of 40 rpm and an autorotation speed of 3500 rpm respectively, and a vacuum degree of -90 KPa to obtain a 6% solid content glue solution;

[0073] S2, adding graphene to the glue in proportion, stirring slowly for 10 min at an orbital speed of 10 rpm and a rotation speed of 1000 rpm respectively, scraping, and then dispersing at high speed for 1 h at an orbital speed of 45 rpm and a rotation speed of 4500 rpm respectively, a vacuum degree of -90 KPa, and then adding carbon nanotubes to the glue in proportion, stirring slowly for 10 min at an orbital speed of 1000 rpm and a rotation speed of 1000 rpm respectively, scraping, and then dispersing at high speed for 1 h at an orbital speed of 45 rpm and a rotation speed of 4500 rpm respectively, a vacuum degree of -90 KPa, and finally adding conductive carbon black to the glue in proportion, stirring slowly for 10 min at an orbital speed of 10 rpm and a rotation speed of 1000 rpm respectively, scraping, and then dispersing at high speed for 1.5 h at an orbital speed of 45 rpm and a rotation speed of 4500 rpm respectively, a vacuum degree of -90 KPa, and obtaining a conductive glue;

[0074] S3. Then, 75% of the lithium iron manganese phosphate is added to the conductive glue, and the mixture is slowly stirred for 10 minutes at an orbital speed of 10 rpm and a rotational speed of 1000 rpm, and the mixture is scraped. Then, the mixture is dispersed at a high speed for 2 hours at an orbital speed of 50 rpm and a rotational speed of 5500 rpm, and the vacuum degree is -90 KPa, and the mixture is mixed evenly. Finally, the remaining 25% of the lithium iron manganese phosphate and 50% of the dispersant and NMP are added to the slurry, and the mixture is slowly stirred for 10 minutes at an orbital speed of 10 rpm and a rotational speed of 1000 rpm, and the mixture is scraped. Then, the mixture is dispersed at a high speed for 3 hours at an orbital speed of 50 rpm and a rotational speed of 5500 rpm, and the vacuum degree is -90 KPa, and the mixture is mixed evenly to obtain a lithium iron manganese phosphate slurry with a viscosity of 3000 to 8000 mPa·s.

[0075] S4, the slurry is adjusted to 350-380g / m 2 The surface density is coated on a 12-14 μm thick carbon-coated aluminum foil, dried, and then coated at 2.2-2.4 g / mm 3 The compaction density is rolled to obtain a lithium manganese iron phosphate electrode.

[0076] The mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry in this embodiment is 58:42. According to the total mass of the powder, the powder is composed of 96.2% lithium manganese iron phosphate (the carbon coating accounts for 1.61wt% of the lithium manganese iron phosphate, the particle size D50 is 407nm, the particle size D90 is 0.848μm, and the specific surface area is 16.506m 2 / g), 1.2% conductive carbon black, 0.4% carbon nanotubes, 0.4% graphene, 0.2% PVDF-1 with a molecular weight of 85.6W, 1.4% PVDF-2 modified with amide and imide groups with a molecular weight of 130.1W, and 0.2% dispersant (Shenzhen Yanyi New Materials Co., Ltd., CGI-D37P);

[0077] Embodiment 2:

[0078] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and based on the total mass of the powder, the powder consists of 96.2% of lithium manganese iron phosphate, 1.2% of conductive carbon black, 0.4% of carbon nanotubes, 0.4% of graphene, 0.3% of PVDF-1, 1.3% of PVDF-2, and 0.2% of dispersant.

[0079] Embodiment 3:

[0080] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and based on the total mass of the powder, the powder consists of 96.2% of lithium manganese iron phosphate, 1.2% of conductive carbon black, 0.4% of carbon nanotubes, 0.4% of graphene, 0.4% of PVDF-1, 1.2% of PVDF-2, and 0.2% of dispersant.

[0081] Embodiment 4:

[0082] The preparation method is consistent with that in Example 1. The mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry described in this embodiment is 58:42. Based on the total mass of the powder, the powder consists of 96.2% of lithium manganese iron phosphate, 1.3% of conductive carbon black, 0.5% of carbon nanotubes, 0.2% of graphene, 0.4% of PVDF-1, 1.2% of PVDF-2, and 0.2% of dispersant.

[0083] Comparative Example 1:

[0084] The difference from Example 1 is that the two-step addition of lithium manganese iron phosphate is changed to a one-step addition. The details are as follows:

[0085] S3. Then, 100wt% of lithium manganese iron phosphate is added to the conductive glue, and the mixture is slowly stirred for 10min at an orbital speed of 10rpm and a rotational speed of 1000rpm respectively, and the material is scraped. Then, the mixture is dispersed at a high speed for 2h at an orbital speed of 50rpm and a rotational speed of 5500rpm respectively, and the vacuum degree is -90KPa, and the mixture is mixed evenly. Finally, 50% of the dispersant and NMP are added to the slurry, and the mixture is slowly stirred for 10min at an orbital speed of 10rpm and a rotational speed of 1000rpm respectively, and the material is scraped. Then, the mixture is dispersed at a high speed for 3h at an orbital speed of 50rpm and a rotational speed of 5500rpm respectively, and the vacuum degree is -90KPa, and the mixture is mixed evenly to obtain a lithium manganese iron phosphate slurry with a viscosity of 3000 to 8000mPa·s.

[0086] Comparative Example 2:

[0087] The difference from Example 1 is that the dispersant is added at one time, that is, only in step (1).

[0088] Comparative Example 3:

[0089] The difference from Example 1 is that the dispersant is added at one time and is only added in step (3).

[0090] Comparative Example 4:

[0091] The difference from Example 1 is that the dispersant is changed to a PVP solution without any modification.

[0092] Comparative Example 5:

[0093] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42. Based on the total mass of the powder, the powder consists of 96.2% of lithium manganese iron phosphate, 1.5% of conductive carbon black, 0.5% of carbon nanotubes, 0.4% of PVDF-1, 1.2% of PVDF-2, and 0.2% of dispersant.

[0094] Comparative Example 6:

[0095] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and based on the total mass of the powder, the powder consists of 96.2% of lithium manganese iron phosphate, 1.5% of conductive carbon black, 0.5% of graphene, 0.4% of PVDF-1, 1.2% of PVDF-2, and 0.2% of dispersant.

[0096] Comparative Example 7:

[0097] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42. Based on the total mass of the powder, the powder consists of 95.95% of lithium manganese iron phosphate, 1.6% of conductive carbon black, 0.3% of carbon nanotubes, 1.3% of PVDF-1, 0.7% of PVDF-2, and 0.15% of a dispersant.

[0098] Comparative Example 8:

[0099] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42. Based on the total mass of the powder, the powder consists of 95.9% of lithium manganese iron phosphate, 2.0% of conductive carbon black, 1.5% of PVDF-1, 0.5% of PVDF-2, and 0.1% of dispersant.

[0100] Comparative Example 9:

[0101] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and based on the total mass of the powder, the powder consists of 95.9% of lithium manganese iron phosphate, 1.2% of conductive carbon black, 0.4% of carbon nanotubes, 0.4% of graphene, 1.6% of PVDF-1, and 0.2% of dispersant.

[0102] Comparative Example 10:

[0103] The preparation method is consistent with that in Example 1, wherein the mass ratio of the powder and the solvent of the lithium manganese iron phosphate slurry is 58:42, and based on the total mass of the powder, the powder consists of 95.95% of lithium manganese iron phosphate, 1.2% of conductive carbon black, 0.4% of carbon nanotubes, 0.4% of graphene, 1.6% of PVDF-2, and 0.2% of dispersant.

[0104] Tests and Results

[0105] The viscosity, solid content, fineness of the lithium manganese iron phosphate slurry prepared in the embodiment and the comparative example, and the peel strength, membrane resistance and flexibility of the lithium manganese iron phosphate pole piece were tested as follows:

[0106] Peel strength test method: Use a universal tensile machine for testing. Cut the electrode into 15*200mm size, then stick the transparent tape horizontally on the bottom of the dried unscaled steel ruler, with the end faces flush; then stick the double-sided tape on the transparent tape, the length is the same as the width of the transparent tape, and the position is centered; finally, stick the test sample on the double-sided tape, with the end faces flush, use the pressure wheel to roll back and forth on the surface of the electrode, then fold the electrode 180° and clamp it on the clamp of the universal tensile machine, stretch it at room temperature at a speed of 50mm / min, select the average value of 30-100mm for the test, peel the positive electrode, and read the test result of the electrode coating peel strength when the electrode current collector and the coating are completely separated.

[0107] Diaphragm resistance test method: Use a DC four-probe tester to measure the electrode resistance, place the electrode on the platform below the tester, adjust the tester bracket studs to make all the needle tips on the four probes form good contact with the electrode, test 15 points for each sample, and take each point 2cm apart, then start the test, and the resistance value displayed after the current value stabilizes is recorded as the electrode surface resistance.

[0108] The test results are as follows:

[0109]

[0110]

[0111] It can be seen from the above table that the conductive glue obtained by the method of the present invention and the method of adding the lithium manganese iron phosphate material to the glue in two steps make the slurry obtained in Examples 1-4 have a viscosity of 6000-7000 mPa·s, a fineness of 8-13 μm, a solid content of 57-60%, and the electrode film obtained in Examples 1-4 has a low resistance of less than 220 mΩ, a high peel strength of not less than 50 N / m, and is opaque when folded twice. Therefore, the electrode prepared by this method has good flexibility, conductivity and bonding properties. It can be seen from Examples 1-4 that the fineness of the slurry is less than 15 μm, indicating that the preparation method disperses the slurry more evenly without particle agglomeration.

[0112] Furthermore, it can be seen from the peel strength of Examples 1-3 that as the PVDF-2 content decreases, the peel strength of the pole piece gradually decreases, which indicates that the complexing groups (polar groups such as carboxyl, hydroxyl, amide, and imide) on the long chain of PVDF-2 can form chemical bonds with the foil, and can effectively enhance the interaction between the positive electrode active material and the current collector, further improve its adhesion, and also improve the flexibility of the pole piece. Furthermore, it can be seen from Examples 1-3 that when the conductive agent added is a composite of conductive carbon black, carbon nanotubes, and graphene and the mass ratio is 3:1:1, as the content of high molecular weight PVDF-2 decreases, the membrane resistance of the pole piece gradually decreases, indicating that the conductive performance of the pole piece has improved, indicating that PVDF-2 has the effect of improving the membrane resistance.

[0113] From the comparison between Example 1 and Comparative Example 1, it can be seen that when the main material is added in one step, the slurry has a larger fineness, there are obvious particle protrusions on the surface of the electrode, and the dispersion is poor, which has an adverse effect on the subsequent capacity, cycle and safety of the battery.

[0114] From the comparison between Example 1 and Comparative Examples 2-4, it can be seen that not adding the dispersant in steps or not using modified PVP has a significant adverse effect on the viscosity, fineness and solid content of the slurry. The dispersion effect of the slurry is poor, and it may even affect the diaphragm resistance and peel strength of the electrode. The flexibility of the electrode also becomes very poor, it is opaque when folded, and there are large areas of pitting on the surface of the electrode, which has an adverse effect on the capacity and cycle performance of the subsequent battery.

[0115] From the comparison between Example 4 and Comparative Examples 5-8, it can be seen that only when the three conductive agents are used in combination, the film resistance of the electrode sheet is low and the conductivity is good. In addition, from the comparison between Example 4 and Comparative Examples 5 and 6, it can be seen that when the other components are the same and only the conductive agent components are different, the three conductive agents are used in combination, not only the conductivity is improved, but also the peel strength of the obtained electrode sheet is improved.

[0116] From the comparison between Examples 1-3 and Comparative Examples 9 and 10, it can be seen that when other components are the same and only the binder component is different, only when small molecular weight polyvinylidene fluoride and large molecular weight modified polyvinylidene fluoride containing carbonyl polar functional groups are used in combination, the obtained slurry has small fineness and high solid content, and the obtained electrode has good conductivity, high peel strength and good flexibility. In particular, from the comparison between Examples 1-3 and Comparative Example 9, it can be seen that the use of modified polyvinylidene fluoride containing carbonyl polar functional groups can significantly improve the peel strength of the electrode.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate slurry, characterized in that: The method comprises the following preparation steps: (1) mixing a binder and a part of a dispersant in a solvent to prepare a glue solution, wherein the binder includes PVDF-1 and PVDF-2, the PVDF-1 is a polyvinylidene fluoride with a molecular weight of 50 to 90W, the PVDF-2 is a modified polyvinylidene fluoride with a molecular weight of 120 to 150W containing a carbonyl polar functional group, and the dispersant is a modified PVP; (2) adding graphene, carbon nanotubes and conductive carbon black as conductive agents to the glue obtained in step (1) in sequence and mixing them to obtain a conductive glue; (3) adding part of the lithium iron manganese phosphate to the conductive glue obtained in step (2) and mixing them evenly; then adding the remaining lithium iron manganese phosphate, dispersant and solvent to the slurry and mixing them evenly to obtain lithium iron manganese phosphate slurry.

2. The preparation method according to claim 1, characterized in that: The carbonyl polar functional group includes one or more of a carboxyl group, a hydroxyl group, an amide group, and an imide group; And / or, the mass ratio of PVDF-1 to PVDF-2 is 1:3-9; And / or, the modified PVP is a polyvinyl pyrrolidone solution modified by a fluorinated acrylic copolymer or an alcoholamine substance; And / or, the mass ratio of the conductive carbon black, carbon nanotubes and graphene is (1-1.5): (0.4-0.5): (0.1-0.5); And / or, the lithium manganese iron phosphate: the carbon coating amount accounts for 1.6-1.8wt% of the lithium manganese iron phosphate, the particle size D50 is 400-500nm, the particle size D90 is 0.8-1μm, and the specific surface area is 15-20m 2 / g; And / or, the lithium manganese iron phosphate slurry includes powder and solvent, and the mass ratio of the powder to the solvent is (55:45) to (63:37).

3. The preparation method according to claim 2, characterized in that: The powder is composed of 94.6-97% lithium manganese iron phosphate, 1-3% conductive agent, 0.8-2% binder, and 0.05-0.4% dispersant by mass percentage; and / or, the viscosity of the modified PVP is less than 200 mPa·s, and the pH is between 10 and 12; And / or, the solvent is N-methylpyrrolidone.

4. The preparation method according to claim 1, characterized in that: In step (1), the solid content of the glue solution is 4-6%; And / or, in step (3), the viscosity of the lithium iron manganese phosphate slurry is 3000-8000 mPa·s.

5. The preparation method according to claim 1, characterized in that: In steps (1) and (3), the mass ratio of the dispersant added twice is (1:3) to (1:1); And / or, in step (3), the mass ratio of the lithium manganese iron phosphate added twice is (3:1) to (1:1).

6. The preparation method according to claim 1, characterized in that: In step (1) and / or step (2) and / or step (3), the mixing includes first slowly stirring the material and then dispersing it at a high speed.

7. The preparation method according to claim 6, characterized in that: The revolution speed and rotation speed of the slow stirring scraper are 10-15 rpm and 1000-1500 rpm respectively, and the time is 5-10 minutes; And / or, the revolution speed and rotation speed of the high-speed dispersion are 40-55 rpm and 3500-5500 rpm respectively, and the time is 1-3 hours.

8. A lithium iron manganese phosphate slurry, characterized in that: The method is prepared by any one of claims 1 to 7.

9. A lithium iron manganese phosphate pole piece, characterized in that: The lithium iron manganese phosphate slurry of claim 8 is coated on a carbon-coated aluminum foil of a certain thickness at a certain surface density, dried, and then rolled at a certain compaction density to obtain the lithium iron manganese phosphate electrode.

10. The lithium iron manganese phosphate pole piece according to claim 9, characterized in that: The surface density is 350-380 g / m 2 ; and / or, the thickness is 12 to 14 μm; And / or, the compacted density is 2.2 to 2.4 g / cm 3 .

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